It is pointed out that, in industries such as microelectronics, certain operations such as the preparation of silicon wafers or chips must be performed in an atmosphere which is free from impurities or dust i.e., in so-called white rooms. This is obtained by ventilating the rooms with air which has been filtered with absolute filters, i.e., filters which stop all dust, even that of an extremely fine nature. At present there are four main methods for maintaining the purity of the air in microelectronics white rooms.
In the first type of white room, the air is introduced into it through the ceiling, and the flow there is of a laminar nature. Personnel works in the room, which also houses equipment. However, efforts are made to reduce the volume occupied by the equipment in the white room. Therefore, the various machines or pieces of equipment are positioned through a tight partition, so that part of them is located in the white room and the remainder in a room where the cleanness requirement is not as high. The main disadvantage of this solution is the need to have a very large clean air volume, and the volume of the air treatment rooms is often three to four times greater than the volume of the actual white room. The installation and operation costs are consequently very high, and the heaviness of the structure does not make it possible to change equipment. Finally, this type of white room suffers from clothing constraints for the personnel.
Because of the foregoing disadvantages of the first type of white room, use is made of another type of white room structure in which the personnel, who generate particles and therefore dust, are replaced by robots. Although the reduction of personnel present in the room makes it possible to obtain a higher cleanness level, robots themselves generate dust, although to a lesser degree than human beings. Moreover, there is only a small reduction in personnel numbers, because the actual handling of the silicon chips only takes up less than 10% of the total working time of employees. Thus, this solution, which requires significant changes in the rooms and to the equipment, is not really commercially viable.
A third solution consists of connecting the different pieces of equipment with the aid of a tube in which the silicon chips circulate by means of an automatic carriage. An ultraclean environment is maintained within the tube and not in the room. Thus, the employees are located in an environment where the degree of cleanness is lower, but which constitutes less of a constraint, whilst the ventilation system installation and operating costs are reduced. However, this solution also leads to disadvantages. Firstly, once the tube has been installed, modifications are only possible with difficulty. Moreover, its connection to the various pieces of equipment or machines make machine modification or changing operations difficult. Finally, the carriages circulating within the tubes are complex, generate dust and are difficult to maintain.
Because of the foregoing disadvantages of the third type of white room, another system has been adopted in which the silicon chips are enclosed in tight boxes for transferring the same into the treatment or processing room. The atmosphere within the tight boxes is of a very high purity level, whereas in the room the purity or cleanness level is of an average nature. The transfer between boxes or between a box and a machine takes place by means of a system of slides or slide valves, which prevent the introduction of external particles or dust into the tight containers. Although this solution has the advantage that the ventilation apparatuses are relatively simple (due to the only average cleanness of the room) and although the use of tight boxes does not lead to all the complications due to the existence of tubes, the fourth solution still suffers from certain disadvantages. Firstly, the silicon chips placed in the boxes are under isostatic environmental conditions, which means that there is little or no air movement. Under these conditions, the particles which could be present in the vicinity of the chips are mainly subject to electrostatic attraction forces developing on the surface of the chips and on the inner walls of the boxes. As the total surface of the silicon chips is much larger than that of the inner walls of the boxes, the particles are mainly deposited on the chips. This is a very significant disadvantage, because when a particle with a diameter of approximately 0.2 micrometer is stuck to a clean surface, it is virtually impossible to disengage it therefrom. Moreover, the system of slides for transferring the chips from one box to the other, or from a box to given pieces of equipment, must be very precise. The operations are accompanied by friction, which can lead to the tearing away of particles, and the latter may then be stuck to the chips. Finally, the various production equipments must be equipped with containers similar to the tight boxes used for transfer, and the installation of these containers is expensive and complex.